(1) Field of the Invention
The present invention relates to a hydraulic control valve, to a device for adjusting blade pitch, and to an aircraft provided with such a hydraulic control valve.
More particularly, the invention relates to a rotary wing aircraft combining at reasonable cost the effectiveness in vertical flight of a conventional helicopter and the high travel speed performance that is made possible by using propulsive propellers and installing modern turboshaft engines.
The invention lies in particular in the technical field of hydraulic control valves for feeding an actuator for controlling the pitch of blades of a rotor.
(2) Description of Related Art
Conventionally, an aircraft has members that are controllable by a pilot, such as the blades of a lift rotor of a rotary wing aircraft or indeed the blades of a propeller.
Using flight controls, the pilot thus controls such members. Nevertheless, the force that needs to be delivered in order to move such members is sometimes very great. Consequently, the linkage connecting a flight control to a controllable member may be provided with a hydraulic system that is sometimes known as a “servo-control”.
Such a hydraulic system has a power actuator co-operating with a hydraulic control valve, the hydraulic control valve being controlled via the flight control.
In conventional manner, an actuator is provided with at least one outer body in which a control piston moves in translation. The control piston may be provided with a power rod. Thus, each control piston defines a retraction chamber and an extension chamber inside each outer body.
Among the various types of actuator, there are single-body actuators having only one body in which a piston moves in translation.
There are also multi-body actuators, each provided with a plurality of bodies, each containing a respective control piston. It is common practice to use a double-body actuator in the field of aviation.
Furthermore, an actuator may be said to be a “single-acting” actuator or a “double-acting” actuator.
A single-acting actuator has a chamber fed with fluid by a hydraulic control valve. A single-acting actuator also has mechanical or electromechanical return means such as a return spring.
In order to move the piston relative to the body in a first direction, the hydraulic control valve feeds fluid to said chamber.
In contrast, in order to move the piston relative to the body in a second direction that is opposite to the first direction, the hydraulic control valve serves to discharge the fluid contained in said chamber. The return means then cause the piston to move in the second direction.
In contrast, a double-acting actuator proposes organizing a first chamber and a second chamber that are separated by a control piston. Each chamber can be fed with fluid by the hydraulic control valve. The travel direction of the piston relative to the body then depends on the fluid pressure in each of said first and second chambers.
In another aspect, an actuator may be an actuator having a stationary body. The actuator body is then fastened to a reference member. A hydraulic control valve can then cause the control piston of the actuator to move.
Conversely, the piston of an actuator may be fastened to a reference member, and a hydraulic control valve can then cause the body of the actuator to move. Such an actuator may be referred to as a “moving body actuator”.
In order to connect the actuator to a hydraulic circuit, at least one hydraulic duct is secured to the body.
The combination of the hydraulic pressure that is usually to be found in the hydraulic duct and the movement of the duct during the movement of the moving body can lead to wear of the hydraulic duct.
Independently of the actuator variant, the actuator may co-operate with a hydraulic control valve.
The hydraulic control valve serves to control the quantity of fluid that is added to or taken from each chamber in the actuator.
A hydraulic control valve comprises a stationary jacket surrounding a movable spool, or indeed a fluid transfer rod connecting the hydraulic control valve to an actuator.
A pilot then controls the position of the spool in the jacket, e.g. in order to put a fluid feed orifice of the jacket into communication with a duct of said transfer rod.
Document FR 2 939 098 shows such a hydraulic control valve having a stationary jacket referred to as “body 23” and a movable spool referred to as “control spool 24”.
Likewise, document FR 2 927 879 describes a control system for varying the pitch of a propeller and including a control valve having a spool. Under such circumstances, that document mentions a control valve having a spool that is movable in a body in compliance with the above-described prior art.
In order to control the pitch of the blades of an airplane propeller, a device is known that comprises a single-acting actuator arranged inside the hub of the propeller. The actuator has a hydraulic chamber co-operating with a piston that is connected to the blades of the propeller.
Since little space is available inside a hub, the actuator is controlled by a transfer rod of a hydraulic control valve, the hydraulic control valve conveying a fluid to the actuator in order to modify the pitch of the blades of the corresponding propeller.
The transfer rod has an outlet orifice that opens out into the hydraulic chamber of the actuator. The transfer rod is also secured to the piston.
Under such circumstances, the hydraulic control valve includes a jacket surrounding a spool. The spool is secured to a control rod that is connected to the flight controls.
Taking action on the flight controls causes the control rod to move, and consequently moves the spool of the associated hydraulic control valve.
As it moves, the spool of the hydraulic control valve allows fluid to flow from the jacket toward the hydraulic chamber via the transfer rod. The increase in pressure in the hydraulic chamber thus causes the piston to move and modify the pitch of the blades. Nevertheless, the movement of the piston also causes the transfer rod to move. The transfer rod is therefore no longer in register with the spool of the control valve, thereby making it possible to stop the movement of the piston.
Such a transfer rod is sometimes referred to as a “repeater” rod insofar as its position is representative of the position of the actuator piston.
Such a propeller control device is conventional in airplanes.
Nevertheless, it should be observed that the distance between the hydraulic control valve and the servo-control is considerable, thereby leading to non-negligible head loss.
Consequently, pitch variation control in the propeller takes place relatively slowly. Thus, the reaction time of the device, between a first moment when the pilot issues an order and a second moment when the order is transcribed, is relatively long.
Such slow control is not a problem for an airplane since the pitch of a propeller is controlled depending on the power of the airplane power plant. Since engine regulation takes place slowly, it is not troublesome for propeller pitch to vary slowly.
However, there also exist aircraft of another type having a rotary wing together with at least one propeller. That propeller may contribute to propulsion, but it also serves to provide the aircraft with yaw control.
In order to maneuver the aircraft in yaw, the pilot makes use of pedals, for example. Unfortunately, in order to respond to a gust of wind or in order to avoid an obstacle, the pilot may need to move the pedals quickly and through a large amplitude.
Since the hydraulic control valve and actuator assembly react slowly, the pilot's order runs the risk of not having any immediate effect.
Furthermore, such a device may also present one of the following drawbacks.
The great length of the transfer rod and the large number of parts mounted within one another lead to a large amount of static interdeterminancy. Furthermore, the spool and the transfer rod must move in translation in accurately parallel manner over a long distance. In spite of very severe dimensional and geometrical manufacturing tolerances, a certain amount of static interdeterminancy remains, with the corollary of a high level of friction.
In addition, the number and the dimensions of the surfaces that are in contact with one another and the degree of static interdeterminancy of the device can lead to high control forces.
In addition, the number of parts used tends to increase the probability of suffering an incident and also to increase the weight of the device.
Lubricating the device may also require a large flow rate of oil.
Document FR 1 260 746 shows a hydraulic control device for a marine propeller that has a spool referred to as a “control spool 6” that slides on a transfer rod under drive from control means.
The movable spool is arranged in a stationary body referred to as a “casing”.
That document FR 1 260 746 does not form part of the technical field of the invention since it is concerned with a marine propeller, where a marine propeller is not subjected to the same constraints as a rotary wing aircraft.
It should also be observed that the spool slides along a fluid transfer rod and along a cylindrical rod given reference “19” that conveys fluid to the movable spool. That device is therefore statically interdeterminant. Furthermore, the device runs the risk of presenting considerable leakage, which may be difficult to accommodate in the context of an aircraft.
Document U.S. Pat. No. 3,812,883 describes a mechanism for preventing a valve from sticking. Grooves are formed in a drum, those grooves having a depth of less than 0.5 millimeters (mm). It should be recalled that grooves are not the same as tapping which, by definition, presents a screw thread.
Document FR 1 528 300 describes a valve having threaded lands for holding a spool hydrostatically in position inside its bore. That document also lies in a technical field that is different from that of the invention.